This company deserves to have its own category because it's a Bitcoin pioneer and likely one of the highest paying companies per CTA (call to action).
Bitcoin companies are able to launch surveys and other tasks via 21.co/paulnatic and pay for each user that completes it.
History of paid tasks in my profile on 21.co/paulnatic
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History of paid tasks in my profile on 21.co/paulnatic
The reward is usually anywhere between $0.10 to $10 per completed task which makes it very attractive even for regular Bitcoin users. In my case, the average profit is around $0.85 in bitcoins per task.
They also randomly send paid offers for copywriting jobs and other things where some skills may be required. For example, a few weeks ago a Bitcoin company was looking for bloggers to write about.